As those skilled in the art are aware, both flow control and structural control devices can be employed on each rotating rotor blade of a helicopter to minimize vibration in flight. The most efficient method of reducing vibration on helicopter rotor blades is through Individual Blade Control (IBC) in which each rotor blade is individually controlled using a flow control or structural control device.
Structural control includes any devices capable of controlling the mass, stiffness or damping of the helicopter blade. The only practical structural control device developed to date is the Active Pitch Link, which is able to control the torsional stiffness characteristics of a blade.
Flow control can be defined as any control technique capable of controlling the aerodynamic loads acting on the blade. Such techniques include Actively Controlled Flap (ACF), Active Twist Rotor (ATR), Actively Controlled Tip (ACT), along with various types of Boundary Layer Suction/Blowing devices. For helicopters, the two most popular techniques have been the Actively Controlled Flap (ACF) and Active Twist Rotor (ATR).
There are a number of major research teams worldwide investigating the feasibility of various active control technologies on helicopter rotor blades. Of the research presently being performed, all research teams consider only one control system per blade. The most popular vibration control systems are of the flow control type with the most popular control system in this category being ACF because of the significantly lower power requirement than ATR. Some prior art systems have applied ACF with two independently controlled flaps on a single blade i.e. two independent control systems of the same type.
However, the problem with applying only one type of control device, especially actively controlled flap (ACF) or active twist rotor (ATR), is that these devices are not very efficient on their own. This is due to the fact that both of these technologies try to actively control the twist (or effective pitch angle) of the rotor blades. This is clearly the goal of a rotor blade employing ATR, but even with ACF it has been shown that a flap is much more efficient when used as a servo-tab than when used as a high-lift device. The goal of a servo-tab is to twist the rotor blade as a result of the flap deflection whereas the goal of the high-lift device is to increase the local rotor blade section lift of a rigid blade.
In order to impose the highest possible twist effect, either as a result of employing ACF or ATR technology, the rotor blade torsional stiffness should be as low as possible. However, the torsional stiffness of a helicopter rotor blade is set to a certain level to avoid excessive deformations due to the aerodynamic loads during operation. This level cannot be lowered by simply making softer blades; otherwise the blades would become too flexible and aeroelastic problems and loss of aerodynamic efficiency would occur.
Therefore, there is a need in the art for some kind of control system allowing the rotor blade torsional stiffness to be lowered whenever the flow control device is actuated.